Generally, adding lead in copper materials would facilitate manufacturing and casting process, so it is widely used for manufacturing faucet structure. However, in recent years, since the price of copper continues to rise and the addition of lead is strictly regulated by law, the processing and material costs of faucets continue to increase. Therefore, the faucet manufacturers gradually shift to use zinc alloy as an alternative material, which is cheaper and easy for surface processing to achieve cost saving in raw materials and processing. However, zinc alloy becomes easy to peel after exposing to water due to oxidation. So, in most faucet structure, a plastic inner tube is disposed to avoid zinc alloy from directly contacting with water to cause oxidation. But, because conventional water outlet duct of the plastic tube has an angle of double concave, it is difficult for the process of injection molding, as well as assembling the structure of the outlet pipe at the faucet, which may reduce the production rate of the faucet. Therefore, there remains a need to develop a water outlet duct that can be fast assembled and easy for injection molding process to substantially increase the efficiency in faucet production and achieve the goal of lowering manufacturing costs.